title: Illusory spirals and loops in crystal growth.
abstract: The theory of dislocation-controlled crystal growth identifies a continuous spiral step with an emergent lattice displacement on a crystal surface; a mechanistic corollary is that closely spaced, oppositely winding spirals merge to form concentric loops. In situ atomic force microscopy of step propagation on pathological L-cystine crystals did indeed show spirals and islands with step heights of one lattice displacement. We show by analysis of the rates of growth of smaller steps only one molecule high that the major morphological spirals and loops are actually consequences of the bunching of the smaller steps. The morphology of the bunched steps actually inverts the predictions of the theory: Spirals arise from pairs of dislocations, loops from single dislocations. Only through numerical simulation of the growth is it revealed how normal growth of anisotropic layers of molecules within the highly symmetrical crystals can conspire to create features in apparent violation of the classic theory.
authors.affiliation: Molecular Design Institute and Department of Chemistry, New York University, New York, NY 10003.
authors.forename: Alexander G
authors.initials: AG
authors.lastname: Shtukenberg
authors.forename: Zina
authors.initials: Z
authors.lastname: Zhu
authors.forename: Zhihua
authors.initials: Z
authors.lastname: An
authors.forename: Misha
authors.initials: M
authors.lastname: Bhandari
authors.forename: Pengcheng
authors.initials: P
authors.lastname: Song
authors.forename: Bart
authors.initials: B
authors.lastname: Kahr
authors.forename: Michael D
authors.initials: MD
authors.lastname: Ward
mesh_terms: D003198:Computer Simulation; D003460:Crystallization; D003553:Cystine / Q000737:chemistry*; D018625:Microscopy, Atomic Force / Q000379:methods*; D008958:Models, Molecular; D008968:Molecular Conformation
chemical_list: D003553:Cystine